Silicon-on-insulator (SOI) wafers are convenient starting material for semiconductor device (chip) fabrication. While the chips can be fabricated on bulk single crystal silicon, the use of the SOI gives advantages of better electrical insulation between individual devices (transistors), extended scaling down capabilities, and better performance of final chips. Two main process technologies for fabrication of SOI are known in prior art, (1) layer transfer, and (2) separation-by-implanted-specie.
The layer transfer consists of the following process steps:    (1) a fragile plane inside of a donor wafer is created; the plane defines the boundary of a layer to be transferred;    (2) an insulator layer is grown on a surface of a handle wafer;    (3) the donor and the handle wafers are initially bonded constituting a wafer assembly;    (4) the donor wafer in the assembly is cleaved along the fragile plane to form an SOI wafer and a leftover portion of the donor wafer;    (5) the initial bonding in the SOI wafer is strengthened.
The separation-by-implanted-specie process consists of the following major steps:    (1) ion implantation 101FIG. 1 of a specie 102 into a silicon wafer 103 creating a buried layer 104 where the specie 102 is mixed with a target material such as semiconductor silicon;    (2) creating a buried insulator layer 105 from the mix layer 104; the implanted specie 102 is chemically active to silicon and can be caused to self organize into a continuous layer of a silicon-specie compound 105.
An advantage of the separation-by-implanted-specie over the layer transfer is process simplicity. It includes 2 processing steps while the layer transfer includes 5 steps.
There is only one version of separation-by-implanted-specie known in prior art that has gained major industrial applications. This is separation by implanted oxygen (SIMOX), described an article “SIMOX” by Julian Blake in Encyclopedia of Physical Science and Technology, 3rd edition, volume 14, pp.805-813, Academic Press, 2002. The implanted oxygen forms an amorphous stoichiometric silicon dioxide SiO2 buried layer inside of the silicon wafer. SIMOX wafers have quality that allowed using them as a starting wafer in commercial chip fabrication.
SiO2 serves acceptably well as an electrical insulator. However, SiO2 is also a thermal insulator. This undesirable thermal resistivity is a main disadvantage of the SIMOX material. Attempts were made in the prior art to create SOI with improved thermal conductivity while keeping simultaneously electrical insulation quality comparable to that of SiO2. Since SiO2 has an extremely low thermal conductivity of 0.014 W/m/K almost any replacement of SiO2 will improve SOI thermal performance.
Implanting of nitrogen was used in the prior art for the first time in the mid 1960s with a purpose to create SO with a silicon nitride layer inside, see Katsutoshi Izumi “Historical overview of SIMOX”, Vacuum, 42 (1991) pages 333-340. A continuous silicon nitride layer was formed; but its quality was not good enough to fabricate chips on these wafers. The process was called SIMNI. A first patent issued for this process for fabricating a single crystalline semiconductor body with a subsurface insulating layer comprising silicon nitride was issued in 1971, Brack U.S. Pat. No. 3,622,382. Since that time attempts to improve quality of the nitride insulator layer to a level enabling chip fabrication on these wafers have not been successful, see for example a book by Jean-Pierre Colinge, “Silicon-on-Insulator Technology: Materials to VLSI”, 2nd edition, Kluwer Academic Publishers, 1997, pp.45-56.
Quality of the buried insulator layer created by the SIMNI process is limited because of the following:    (1) nitrogen bubbles appear in the middle of the buried insulator layer    (2) high leakage currents through the buried insulator layer    (3) high interface states density occurs at the silicon—buried insulator interfaces.
Causes for the listed above detrimental features of SOI with Si3N4 formed with the SIMNI process are the following:    (1) nitrogen has low diffusivity in stoichiometric silicon nitride Si3N4;    (2) Si3N4 tends to crystallize at temperatures exceeding 800° C.
Here we describe in more detail why the low diffusivity and crystallization of Si3N4 prevents fabrication of high quality SOI with the SIMNI process. The low nitrogen diffusivity in stoichiometric silicon nitride Si3N4 further limits the thickness of the continuous Si3N4 layer that can be formed. If a plain silicon wafer is thermally nitridized (i.e, annealed in nitrogen ambient), nitrogen first chemically reacts with surface silicon atoms thus starting a Si3N4 layer. Further growth of the Si3N4 layer is limited by the diffusion of nitrogen through the already grown Si3N4 layer toward silicon—Si3N4 interface. The maximum thickness of Si3N4 that can be thermally grown is about 30 Å which is too thin for most foundry processes. To obtain this maximum thickness, nitridation should be performed at the highest possible temperatures (i.e., approaching silicon melting temperature). By comparison, the maximum thickness of a surface SiO2 layer obtained by thermal oxidation is about 1 micrometer. This is 300 times thicker. The SiO2 thickness limited by the same phenomenon: diffusion of a reactant through a grown compound, i.e., oxygen through SiO2. Creating a buried insulator layer from a mix during the second step of either SIMM or SIMOX is similar to the processes of thermal nitridation and oxidation in that both processes are limited by diffusion of the implanted specie through the already grown silicon compound. Finally, the maximum thickness of a silicon compound layer obtained by thermal treatment is the same for both cases: for the surface, and for the internal silicide formation. That means that we cannot expect to get a high quality buried Si3N4 layer thicker, than 2×30 Å=60 Å. The factor 2 appears because the nitride grows in 2 directions, toward bulk, and toward surface of the wafer. This consideration gives us a criterion for choosing nitrogen implantation dose in SIMNI. To obtain less than 60 Å Si3N4 layer, nitrogen implantation dose should not exceed 1017 cm−2 for N+ implantation, or 5×1016 cm−2 for N2+ implantation. Attempts to use this low dose are known in prior art. They, however, resulted in creation of Si3N4 islands inside of silicon instead of getting a continuous Si3N4 layer. The reason is that the implanted specie (nitrogen here) is distributed along a depth in the target due to a random character of stopping of ions in the solid. The depth distribution of the implanted species is characterized phenomenon known as vertical struggling. This struggling depends on masses of atoms of target lattice and implanted specie, and on the energy of implantation. In the case of nitrogen implanted into silicon, the struggling is about 250 Å at lowest reasonable energy of implantation (20 keV) and it increases with implantation energy reaching 1500 Å at 800 keV. Energy that is lower than 20 keV cannot be used because in an SOI process it results in a surface nitride layer instead of the desirable buried layer. One can expect to obtain a continuous buried layer even in the case when the final thickness of the internally grown layer is much less than the struggling lengths. It requires collecting of the distributed implanted specie in and near a single defined plane inside of the wafer. This phenomenon is observed for some cases (for example, for hydrogen implanted into silicon, and for oxygen implanted into silicon). Hydrogen and oxygen collects at a plane of maximum vacancy-type defects in silicon. However, the nitrogen implanted into silicon does not show this self confining feature. Success of SIMOX can be thus explained by the (1) SiO2 layer thickness exceeding struggling, and (2) self-confinement of the struggled implanted oxygen toward SiO2 layer nucleated at vacancy peak.
Another feature of Si3N4 is that it is prone to an amorphous-to-crystalline transition at quite low temperatures starting from 800° C., see for example V. S. Kaushik, A. K Datye, D. L. Kendall, B. Martinez-Tovar, D. S. Simons, D. R. Myers, “Kinetics of silicon nitride crystallization in N+-implanted silicon”, Journal of Materials Research, 1989, Vol. 4, pp. 394-398. By comparison, amorphous SiO2 starts to crystallize at much higher temperatures ˜1500° C. To transform the nitrogen/silicon mix into Si3N4, however, a temperature exceeding 1200° C. is required, as explained above. Therefore the nitride SOI obtained with SIMNI has the buried insulator layer consisting from polycrystalline Si3N4. The polycrystalline layer has high current leakage along its crystal grain boundaries. It does not show desirable electrical insulative properties.
Attempts to form SiC, AIN, AM2O3 and other SOI with separation-by-implanted-species are known in the prior art. These attempts have been even less successful than with SIMNI and did not result in creation of a desirable continuous buried insulator layer.
Silicon nitride has thermal conductivity of about 0.3 W/m/K while silicon dioxide has thermal conductivity 20× lower of 0.014 W/m/K. Attempts were made in prior art as to make SOI with oxynitride SiNxOy buried layers. This process called SIMON (see above cited Colinge book) results in a thermal conductivity of the oxynitrides close to that of SiO2. SIMON does not present advantages in thermal performance of SOI.
Hydrogenated silicon nitrides do however have a thermal conductivity approaching that of amorphous stoichiometric silicon nitride, i.e., 0.3 W/m/K. The art would therefore benefit from an improved method for hydrogenated nitride SOI fabrication.